The invention relates to instruments for measuring the evaporation of a liquid in an environment. Specifically, the invention relates to a device called an atmometer which models the water vapor diffusion resistance and the albedo of field crops.
The '585 patent describes an atmometer whose evaporation surface is covered with a green canvas material which simulates the albedo and diffusion resistance properties of a crop in order to provide an accurate estimation of evapotranspiration. A check valve in this instrument helps prevent precipitation from adversely affecting the gross evapotranspiration estimate.
Although the canvas covering has proven to be an effective simulator of crop canopy resistance, and a reliable check valve has been developed to prevent inflow of water from precipitation, some problems remain.
The canvas, which measures 0.020 inches thick, is somewhat stiff and bulky. When it is tied down to the flat evaporation surface of the atmometer there may be an air space of from 0.010 to 0.020 inch between the canvas and the wet surface of the porous ceramic. The air space adds significantly to vapor diffusion resistance and has been found to cause as much as an 8% decrease in evaporation rate. Also, the bulk of the canvas fabric will saturate with water during rain or heavy dew. To some extent this simulates natural wetting of a crop, but the amount is excessive. After precipitation there is an unnaturally long delay before water is again drawn from the instrument by evaporation because the saturated canvas must first dry.
The check valve is designed to remain tightly closed until a positive differential pressure threshold is reached. Because of this differential pressure, and also because of the weight of water between the elevated ceramic evaporation plate and its reservoir, a sub-atmospheric pressure exists in the cavity of the evaporation plate. If there is a bubble of air in the cavity, it may provide space for water from precipitation to enter the cavity even if the check valve is functioning properly.
A method tried by others to avoid problems caused by precipitation has been to shelter the evaporation surface by a transparent sheet that is high enough to allow free circulation of air and clear enough not to block solar radiation to the surface significantly. This method has proved cumbersome and inadequate in practice.
Canvas covers of the type described in the '585 patent may come treated with water repellents. In my experience they are not sufficiently waterproof. For example, they tend to become saturated in driving rain. Furthermore, since they are in contact with the evaporating surface below, they tend to wick water down onto that evaporating surface where it is absorbed. This absorbed water produces significant error in the evaporation measurement. Moreover, repellents rapidly lose their effectiveness due to weathering.